Film movies are shot at a frame rate of 24 Hz (e.g. 24 frames-per-second fps). Although advanced digital televisions support 24 fps and can display movies natively, analog televisions and many digital televisions cannot. As a result, movie material is converted to either 30 interlaced frames (30i) or 60 progressive frames (60p) by display devices (e.g. DVD player or TV). To up-convert 24 Hz to 60 Hz, four progressive movie frames are converted to five interlaced frames or 10 progressive frames. The conversion process is known as “3:2 pulldown” or “2:3 pulldown.”
During playback as video, the 60 Hz interlaced video stream is generated. When converting the film to interlaced video, each movie frame is turned into two or three video fields, which creates an uneven distribution of fields. In addition, a movie frame may get split into odd and even video fields where an odd field is the frame with only the odd pixel lines and an even field is the frame with only the even pixel lines. Typically, the odd and even video frames of an interlaced video are called fields.
Telecine is a term for converting movie content to TV/video form. Cadence correction is the restoration of the original frame sequence in video material. Cadence correction is commonly used to reverse the telecine process that converted movies shot on film into interlaced video back into the original 24 frames-per-second progressive film sequence. Cadence correction is part of deinterlacing, which in order to apply the appropriate algorithm, must determine if the original movie was shot with a film or video camera. It is also called cadence detection, film mode detection, reverse telecine, inverse telecine, and reverse 3:2 pulldown. Deinterlacers process interlaced video and convert the video to progressive form. Deinterlacers do not process input streams that are already in progressive form.